1. Field of the Invention
The present invention relates to display devices and, more particularly, a source of illumination for liquid crystal displays.
2. Background
Liquid crystal devices (LCDs) function as effective modulators of light, so that displays based on these devices require a separate source of luminance. Transmissive LCD displays use an integral backlight placed behind the LCD. These backlights usually fall into one of two generic types.
Edgelit lightpipes are used in applications such as notebook computers, where backlight depth is a premium parameter, and high luminance is not required. Direct backlit LCDs, using lamps mounted in a reflector housing and behind a diffusing screen (to hide the lamp shape), are used where high luminance is more important. These systems are usually somewhat bulkier than those using edgelit lightpipes.
Fluorescent lamps are commonly used as the illumination source in LCD backlights, since they have the requisite properties of high efficiency, long life and good color balance. Small diameter straight fluorescent tubes are often used as the edgelight illuminators, and single or multiple straight or serpentine fluorescent tubes are used as the direct backlight illuminators.
In this latter case, increasing the number of lamps and/or incorporating longer serpentine lamp lengths (more channel "legs") increase light output and uniformity (thereby permitting thinner structures). On the downside, greater numbers of lamps increase driver complexity and cost, and increased serpentine lamp length reduces dimming range.
Fluorescent lamps are filled with a mixture of an inert gas (typically Argon) and Mercury vapor, at low pressure. Electrons emitted by the lamp cathodes are used to ionize the gas, and a drive waveform applied to the electrodes causes a net charge transport through the lamp medium (current flow).
The Mercury vapor ions radiate in the ultraviolet (UV) and it is this UV radiation which excites the lamp phosphors, causing them to emit light in the visible spectrum. Lamp luminance is typically controlled either by varying the frequency of the drive waveforms to control the net current flow, or by varying the conduction duty cycle by controlling the energy in each drive pulse.
In any case, uniform emission is difficult to maintain over a very wide dimming range (particularly at low luminance levels). This problem is worse for longer arc path lengths.
Dimming is not usually a major concern for LCDs intended for indoor or office applications, or other controlled ambients. It becomes increasingly important for outdoor applications such as ATM machines, or for cars, aircraft or other vehicles where the display is likely to be viewed under conditions ranging from bright sunlight to total darkness. By far the most demanding application in terms of dimmability is in the cockpits of tactical military aircraft, where a dimming range in excess of 4000:1 is usually required.
While techniques for dimming fluorescent lamps are well known, the ability to maintain a stable uniform luminance output at extremely low luminance levels is an intrinsic limitation of the fluorescent lamp technology, and one which is further complicated by long arc channel lengths.